Conventionally, a construction machine having an exhaust gas processing device is known. The exhaust gas processing device purifies nitrogen oxides in exhaust gas discharged from an engine. The exhaust gas processing device causes a reduction reaction of nitrogen oxides in the exhaust gas and a reduction agent to occur using a reduction catalyst, for example, to decompose the nitrogen oxides into water and nitrogen. The exhaust gas processing device includes a NOX purification device that is disposed on an engine and has a reduction catalyst therein, a urea water tank that accommodates urea water as a reduction agent to be supplied to the NOX purification device, and a urea water injection valve that is attached to the NOX purification device so as to inject the urea water in the urea water tank into the NOX purification device (for example, see Japanese Unexamined Patent Publication No. 2012-237232). In the exhaust gas processing device, the urea water injection valve injects urea water into the NOX purification device. Moreover, the exhaust gas processing device causes a reduction reaction between nitrogen oxides in the exhaust gas and an ammonia generated from the urea water by a reduction catalyst in the NOX purification device. In this way, the exhaust gas processing device purifies nitrogen oxides in the exhaust gas.
The urea water injection valve of the exhaust gas processing device, that is, the reduction agent supply unit, is attached to the NOX purification device through which hot exhaust gas passes. Due to this, the temperature of the reduction agent supply unit is likely to increase with the heat of the exhaust gas. When the temperature of the reduction agent supply unit increases, the temperature of the urea water passing through the reduction agent supply unit also increases. Due to this, in the exhaust gas processing device, an excessively large amount of ammonia may be generated due to hydrolysis of urea water. In such a case, after the exhaust gas processing device causes a reduction reaction of the nitrogen oxides and ammonia to occur in the NOX purification device, the ammonia may remain in the NOX purification device, as a result, the NOX purification device may corrode. Due to this, it is desirable to provide an exhaust gas processing device capable of suppressing an increase in the temperature of the reduction agent supply unit and a construction machine having the exhaust gas processing device.